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<html lang="en">
    <head>
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <title>Document</title>
    </head>
    <body>
        <script>
            function queryData(url) {
                return new Promise(function(resolve, reject) {
                    var xhr = new XMLHttpRequest();
                    xhr.onreadystatechange = function() {
                        if (xhr.readyState != 4) return;
                        if (xhr.status == 200) {
                            resolve(xhr.responseText);
                        } else {
                            reject('出错了');
                        }
                    };
                    xhr.open('get', url);
                    xhr.send(null);
                });
            }
            // queryData('http://localhost:3000/data').then(
            //     function(data) {
            //         console.log(data);
            //     },
            //     function(data) {
            //         console.log(data);
            //     }
            // );
            // 发送多个Ajax请求并保证顺序
            queryData('http://localhost:3000/data')
                .then(function(data) {
                    console.log(data);
                    return queryData('http://localhost:3000/data1');
                })
                .then(function(data) {
                    console.log(data);
                    return queryData('http://localhost:3000/data2');
                })
                .then(function(data) {
                    return console.log(data);
                });
        </script>
    </body>
</html>
